1. Field of the Invention
The present invention relates to a technique for a multimedia encoding and decoding, and in particular to a technique of compression coding for carrying out motion prediction coding between frames by expressing a dynamic color image by a brightness component and a color difference component.
2. Description of the Related Art
An information volume of a video is generally very large, making it highly costly if media accumulation or network transmission is carried out utilizing the original form of the video. Because of this, the development and standardization for compression coding of a video in an encoding or a decoding system has conventionally been tried widely. Representative examples include MPEG-1, MPEG-2, MPEG-4 and AVC/H.264.
The above standards adopt what is referred to as “inter-frame motion prediction coding”. Between-frame motion prediction coding is a method of searching for a section of high correlation between frames and then coding the positional difference (i.e., a motion vector) and a pixel value difference (i.e., a prediction error) between a frame and the following frame. This makes it possible to accomplish high compression efficiency because the correlation between sequential frames is generally high in a video, making the pixel difference value become smaller as compared to the original pixel value.
Search and/or determination methods for a motion vector are not specified by the above noted standards. Therefore, the image quality performance of a coding apparatus greatly depends on the prediction accuracy of a motion vector. Meanwhile, the process volume of detection of a motion vector greatly increases with the size of the search range or the number of search positions, and therefore a tradeoff exists between image quality and the circuit size and/or a power consumption of an apparatus.
Only a brightness component is generally used for a detection of a motion vector. This is because the resolution of color difference is finer as compared to that of a brightness component, limiting the degradation of a subjective image quality to a minimum if only the resolution of the color difference part is reduced. This explains why the information ratio Y:Cb:Cr commonly uses a 4:2:0 form, where the Y is for a brightness component and the Cb and Cr are for color difference components in a dynamic color image.
Such a motion vector detection method using only a brightness component does not have difficulties in handling a common image, whereas it allows for degraded prediction accuracy of a motion vector in the case of an image in which a brightness component is uniform (i.e., nonexistence of a texture) and also a texture exists only in a color difference component, possibly resulting in great degradation of a subjective image quality.
The next description is of the above noted problem by referring to FIG. 1.
Screen examples shown in FIG. 1 show a state of a red circular foreground body (which is indicated by a shaded circle) moving from the right to left. Note that, for the purposes of this example, it is assumed that brightness components of the foreground and background are respectively constant, with only color differences being different between them in the screen examples.
Referring to FIG. 1, the row (a) shows an image component of the current frame, and the row (b) shows an image component of the frame at a unit time prior to the current clock time. The dotted line arrow shown in FIG. 1 is the correct motion vector in the example shown by FIG. 1. It is assumed, however, that the motion vector is incorrectly detected as the solid line arrow shown in the drawing due to the brightness component being uniform across the entirety of the screen in the example shown in the drawing. The row (c) of FIG. 1 shows an image component of the frame after compensating a motion that is obtained as a result of the erroneous detection.
A post-motion compensation frame is obtained by subtracting a motion compensation frame from an image component of the current frame. Note that the motion compensation frame is the frame at a unit time prior to the current clock time spatially moved by the amount of a motion vector. As described above, since the motion vector is erroneously detected as the solid line arrow, the motion frame in the example of FIG. 1 is the red foreground body in the frame shown in the row (b) that is moved in the direction opposite to the aforementioned solid line arrow. Having subtracted the motion compensation frame from the current frame shown in the row (a), the post-motion compensation frame allows a cyan-colored (i.e., the complementary color of red) body image (which is expressed by the black circle in FIG. 1) to appear in addition to the red foreground body.
As such, the use of a degree of correlation sometimes results in detecting an erroneous motion vector instead of being able to obtain a correct one in the case when a brightness component is uniform. As a result, the information volume of a post-motion compensation frame (i.e., the frame in the row (c)) becomes larger when compared to that of the original image frame (i.e., frames in the row (a)) as is apparent from FIG. 1. At this juncture, a quantization parameter (i.e., a resolution of a quantization) needs to be increased in order to keep a limit on a bit rate (i.e., a generating information volume), which has been predetermined, resulting in increasing the quantization error of a color difference component, however. Moreover, because the brightness component is uniform in this case, the quantization error of the color difference becomes visually apparent, thereby greatly degrading subjective image quality.
Related to this problem, a Laid-Open Japanese Patent Application Publication No. 08-102965, for example, has disclosed a technique using the total of an accumulation value of the prediction error of a brightness component plus that of the prediction error of a color difference component to determine an accuracy evaluation value of a motion vector when searching a motion vector.
Another conceivable approach to the problem may be to minimize a quantization parameter (i.e., an adaptive quantization) of a small block in which an occurrence of an image degradation is predicted. Techniques for a common adaptive quantization have respectively been disclosed by Laid-Open Japanese Patent Application Publications Nos. 07-107481, 2001-522174 and 2002-64829, for example.
Among these disclosed techniques, the one disclosed in Laid-Open Japanese Patent Application Publication No. 07-107481 is configured to determine a quantization parameter from the activity, evenness, and degree of buffer accumulation of a block. The technique disclosed in Laid-Open Japanese Patent Application Publication No. 2001-522174 is designed to minimize the quantization parameter of a block having a predetermined color such as a human flesh color. Finally, the technique disclosed in Laid-Open Japanese Patent Application Publication No. 2002-64829 is designed to enlarge the information volume of a block referred from a block of another frame.
The technique disclosed in Laid-Open Japanese Patent Application Publication No. 08-102965 always uses a color difference component in searching for a motion vector and therefore the arithmetic operation volume increases when compared to the instance using a brightness component only. This presents a critical problem when implementing a coding apparatus, such as digital camera, that needs suppressing of a circuit size and power consumption.
Furthermore, the techniques disclosed in Laid-Open Japanese Patent Application Publications Nos. 07-107481, 2001-522174 and 2002-64829 also suffer from a problem when producing a prediction of a small block in which a degradation of an image occurs depending on an image.